This invention relates to a process for producing ethanol from methanol, carbon monoxide and hydrogen.
It was known in the prior art that ethanol was produced from methanol, carbon monoxide and hydrogen by using a catalyst comprising a cobalt salt as a main component and iodine, bromine, an iodine compound or a bromine compound and optionally a ruthenium compound or an osmium compound. According to this prior methods, many by-products, such as dimethyl ether, methyl ethyl ether, acetaldehyde, dimethoxy ethane, acetic acid, methyl acetate, ethyl acetate, methyl formate and compounds having C.sub.3 or more were produced together with ethanol. That is, selectivity to neat ethanol was insufficient in the prior method.
Use of iodine, bromine, an iodine compound or a bromine compound is critical in the prior method, so corrosion-resistant, expensive material must be used as a material for equipment, when the method is carried out on an industrial scale. In addition, when a cobalt compound, an iodine compound and a ruthenium compound coexist, insoluble material is separated out in the mixture of the starting material and the catalyst and liquid mixture containing the insoluble material can not be fed smoothly into high pressure equipment.
It is desirable that catalysts exhibiting high selectivity to ethanol and not containing iodine, bromine or compounds of these elements be developed for use in a process for industrially producing ethanol from methanol, carbon monoxide and hydrogen.
Catalysts not containing iodine, bromine, an iodine compound or a bromine compound have been proposed for the above purpose, as exemplified in U.S. Pat. No. 4,168,391 patented on Sept. 18, 1979 by Slinkard et al. U.S. Pat. No. 4,168,391 teaches a process for producing ethanol which comprises reacting methanol, carbon monoxide and hydrogen in the presence of a dicobalt octacarbonyl catalyst and an oxygen-containing compound, such as an ester, an alcohol, a ketone, or an ether to form ethanol in reactivity of methanol of 53% and selectivity to ethanol of 69%.
British Patent Application No. 22490/1977 filed on May 27, 1977 teaches reacting methanol, carbon monoxide and hydrogen in the presence of a cobalt compound and a carboxyl group-containing solvent, such as an ester or an organic acid to form ethanol in reactivity of methanol of 36% and selectivity to realizable ethanol of 82.2%.
British Patent Application No. 6291/1978 filed on Feb. 17, 1978 teaches reacting methanol, carbon monoxide and hydrogen in the presence of a cobalt compound and an oxygen-containing compound, such as an aldehyde, a ketone, an alcohol or an ether to form ethanol in reactivity of methanol of 35.1% and selectivity to realizable ethanol of 72.6%.
However, when methanol is reacted with carbon monoxide and hydrogen in the presence of any one of these known catalysts, by-products, such as dimethyl ether, methyl ethyl ether, acetaldehyde, dimethoxy ethane, methyl formate, methyl acetate, ethyl acetate and other compounds of C.sub.3 or more other than ethanol (object product) are formed, so selectivity to neat ethanol is low and complicated operation is necessary for separating ethanol from the reaction mixture.
The present inventors repeated the methods disclosed in the above references and always obtained results inferior to the results given in the above references with respect to selectivity to ethanol. Particularly, when an organic acid is used as a solvent as disclosed in British Patent Application No. 22490/1977, the ethanol obtained and the organic acid form an ethyl carboxylate, so the amount of neat ethanol is small. In addition, most of the methanol, which is a starting material, forms a methyl carboxylate with the organic acid. So, a step for separating ethanol from the mixture of ethanol and methanol obtained by hydrolyzing the ethyl carboxylate and the methyl carboxylate is necessary. Therefore, the method is not satisfactory from an industrial point of view.